1. Field of the Invention
The present invention relates to a process for controlling the properties of white liquor used by the preparation of boiling liquor for use in the sulphate process, the electric conductivity of the liquor being measured by said process after the causticization.
By the so-called kraft process or sulphate process pulp is prepared by digesting chips of wood in a strongly alkaline liquor mainly containing NaOH and Na.sub.2 S. During the pulping, the content of NaOH of the liquor is consumed, said content being relatively high at the beginning, whereas its content of Na.sub.2 S remains substantially unchanged.
The spent pulping liquor is called black liquor and contains, inter alia, the dissolved lignin in addition to the residue chemicals. The black liquor is concentrated and subsequently burnt in a steam boiler, whereby its content of energy is utilized. In the bottom of the boiler, the combustion products are collected in the form of a melt mainly consisting of Na.sub.2 S and Na.sub.2 CO.sub.3.
The melt is introduced into a tank with water (thin liquor), wherein it is dissolved. The resulting liquor is called green liquor. The green liquor always contains a small amount of NaOH which may vary a great deal. The green liquor is characterized by having a high content of Na.sub.2 CO.sub.3 and a small amount of NaOH. In order to re-form the boiling liquor quicklime is added to the green liquor in a slaker, in which the following reactions take place:
Slaking: EQU CaO+H.sub.2 O.revreaction.CA(OH).sub.2 ( 1)
Causticization: EQU Ca(OH).sub.2 +Na.sub.2 CO.sub.3 .revreaction.2NaOH+CaCO.sub.3.dwnarw.( 2)
The reactions proceed in parallel towards a state of equilibrium and are more or less displaced to the right. The liquor formed by the causticization is called white liquor.
The content of Na.sub.2 S of the green liquor does not take part in the process, but is retrieved in the white liquor. When the amount of water consumed by the reaction (1) is ignored (about 2%), the content of Na.sub.2 S of the white liquor is equal to the content of Na.sub.2 S of the green liquor. In order to permit a completion of the processes, the liquor is transferred from the slaker to the first of a row of causticizers with stirring. The contents of the first causticization vessel or causticizer are transferred into the next vessel and so on until an overflow is established by the causticizers being positioned in gradually lower heights. The number of causticizers may vary.
Upon termination of the reaction, the causticized liquor (white liquor) is separated from the calcareous silt. Subsequently, the white liquor is used, optionally after additional clarifying, for the preparation of a new pulping liquor. The white liquor and consequently the pulping liquor always contain a small amount of unreacted Na.sub.2 CO.sub.3. The white liquor is characterized by containing a high amount of NaOH and a small amount of Na.sub.2 CO.sub.3. The calcareous silt is flushed for liquor residues, dehydrated, and burnt in a rotary kiln whereby the calciumoxide (quicklime) necessary for the causticization is re-formed. The wash water is called thin liquor and is utilized in the dissolving tank for the melt formed from the black liquor, whereby the content of liquor residues of the thin liquor is reused.
As it will appear, the chemicals circulate in two circuits, viz. one for the sodium (the digesting process--the evaprotation--the combustion--the dissolving--the separation from the white liquor--the washing out--the dehydration--the burning). The unavoidable loss of chemicals is as far as the sodium is concerned replaced by addition of Na.sub.2 SO.sub.4 to the concentrated black liquor. At the combustion, the major part is reduced to Na.sub.2 S, which results in the name the sulphate process. A small amount of NaOH may furthermore be added to the white liquor. As far as the calcium is concerned, quicklime CaO may be added at the outlet of the rotary kiln, or lime CaCO.sub.3 may be added at the inlet of the rotary kiln.
The white liquor and the green liquor may be characterized by some quantitites, the defintion of which is recommended to be used by central laboratories of which for wood or paper pulp in Scandinavia, and which for instance are mentioned in SCAN-N 2:63, whereby the statement of the various chemical substances is to be understood as the concentration of the compound in question, calculated as g of NaOH/l:
Active alkali AA=NaOH+Na.sub.2 S PA1 Efficient alkali EA=NaOH+0.5Na.sub.2 S PA1 Total alkali=all alkali salts. PA1 Total titratable alkali TTA=NaOH+Na.sub.2 S+Na.sub.2 CO.sub.3
The degree of causticization (in white liquor) ##EQU1##
The sulphidity (in white liquor) ##EQU2##
The degree of reduction (in green liquor) ##EQU3##
Furthermore the following can be mentioned: The reaction of carbonate: Na.sub.2 CO.sub.3GR --Na.sub.2 CO.sub.3HV g/l
The degree of the reaction of carbonate: ##EQU4##
The control of the causticization process may choose the calcium cycle, the sodium cycle or both the calcium cycle and the sodium cycle as starting point.
As far as the sodium cycle is concerned, the difficult steps in the process are centred about the combustion of the concentrated black liquor and the causticization process. By the causticization particularly the control of the addition of quicklime is difficult. This is partly due to the great time lag between the addition of quicklime and the filtration (2-3 hours) of the finished liquor, partly due to the variability of the quicklime both with respect to its reactivity (the slaking velocity) and with respect to its content of active lime. However, the difficulty in controlling the addition of quicklime especially depends on the fact that it has not previously been possible continuously to measure or determine process relevant parameters to be used for the required control.
2. Description of the Prior Art
Previously, the causticization process was often controlled by means of manual regulation of the feeding of quicklime on the basis of laboratory analyses of the white liquor immediately after the slaker and optionally of the green liquor, whereby it was tried to maintain the degree of causticization (in the white liquor) at a predetermined value. This procedure is encumbered with the drawback that it is necessary to wait so long for the result of the analysis that in general it is too late to establish the necessary restoration of the causticization process. Attempts to restore the causticization process may easily involve for instance undue calcareous concretion causing a poorer filtratability and a too high content of calcium in the white liquor whereby filters, pipes, pumps, boilers etc. are calcified, cf. K. Kinzner "Untersuchungen zur Kaustizierung von Grunlaugen," in Proceedings of the Symposium in Recovery of Pulping Chemicals, Helsingfors 1968, page 279. By keeping the specific gravity and consequently the TTA-value of the green liquor constant, it is possible to influence the causticization process positively concerning keeping of the degree of causticization. However, it is not possible to take into account the considerable variation of the quality of the lime by keeping the specific gravity of the green liquor. In addition, the composition of the green liquor may vary considerably irrespective of the fact that the specific gravity and consequently the TTA-value are kept constant. The content of NaOH of the green liquor may for instance vary considerably depending on the amount of water which it is necessary to add to the dissolving tank in addition to the thin liquor in order to dissolve the melt resulting from burning of the concentrated black liquor.
It is known to control the causticization process by means of an automatic titrator for the determination of the content of Na.sub.2 CO.sub.3 in the green liquor and the white liquor, two temperature measurings being simultaneously performed, viz. a temperature measuring of the green liquor immediately before the slaker and a temperature measuring of the contents of the slaker. The slaking (process 1) involves generation of heat, whereas the causticization (process 2) involves no significant heat content change. The rise in temperature of 10.degree.-15.degree. C. renders it possible to calculate the amount of calcium hydroxide available for the causticization. This calculation renders it possible to control the addition of quicklime, said control being performed in preparation for a constant degree of causticization. An automatic titrator is, however, expensive and must be kept up with analysis reagents, cleaned, and altogether controlled with respect to its function, and it uses time for performing an analysis. The measurings achieved are therefore delayed relative to the moment the necessary control signals should have been given. Consequently, these measurings and the registration thereof cannot be considered on-line. The rise in the temperature measured is relatively modest, and in order to achieve an accurate figure of the amount of calcium hydroxide available for the causticization, the temperatures must be measured individually with great accuracy.
By another known measuring method, the conductivity of the white liquor measured after the slaker is used as a measurement of the degree of causticization, and this measurement is made the basis of the control of the amount of added quicklime. However, the electric conductivity of a solution depends on all the electrolytes present in the elecrolytic solution in question, on their concentration, and on the temperature, since in practice it is always necessary to temperature compensate a conductivity measuring to some reference temperature. The conductivity of the white liquor depends not only on the composition of the liquor concerning NaOH or Na.sub.2 CO.sub.3 (the degree of causticization), but also on the content of Na.sub.2 S, and the concentration is of particular importance. Therefore a measuring of the conductivity solely of the white liquor does not permit a good determination of a parameter, on which it is possible to base a control of the degree of causticization.
As it is known, an aqueous solution of NaOH possesses a much higher conductivity than an aqueous solution of Na.sub.2 CO.sub.3 having the same concentration. An aqueous solution of Na.sub.2 S having the corresponding concentration possesses a conductivity between the conductivity measured for the NaOH and the Na.sub.2 CO.sub.3 solution, respectively.
Furthermore the solution having the highest amount of NaOH among the aqeuous solutions of mixtures of electrolytes containing NaOH, Na.sub.2 S, and Na.sub.2 CO.sub.3 possesses the highest conductivity provided the content of Na.sub.2 S is constant, the sum of the amounts of substance of said solutions being equal, e.g. calculated as g of NaOH/l or as g of Na.sub.2 O/l.
Concerning a green liquor and the white liquor derived therefrom by causticization, the sum of the amounts of electrolytes, calculated as g of NaOH/l, is equal in the green liquor and in the white liquor, the content of Na.sub.2 S not being influenced by the causticization process. Since the white liquor contains more NaOH than the green liquor from which it is derived, it also possesses a higher conductivity than the green liquor, and this recognition is the basis of the invention.